A recent problem for a liquid crystal display device (hereinafter often abbreviated to LCD) requires 1 that a display should have a quick electro-optical response in order to display a moving image and 2 that a display material should have a wide nematic range (a high clearing point) in order that it can be used in outdoors. Various attempts have been made in various liquid crys-tal display modes to solve the problem. Among those display modes, an active matrix liquid crystal display (hereinafter often abbreviated to AM-LCD) mode employing a thin film transistor (hereinafter often abbreviated to TFT) has begun to attract much attention as one of the most practical flat display, since it has enabled a fine color display.
This AM-LCD employs as a driving mode a TN display mode in which aligning directions of liquid crystal molecules on a pair of substrates are twisted by 90 degrees each other. As G. Bauer reported in Cryst. Liq., 63, 45 (1981), it is required in this TN display mode to set a product .DELTA.n.d (.mu.m) of a cell gap (hereinafter often abbreviated to d) and an optical anisotropy value (hereinafter often abbreviated to .DELTA.n) of liquid crystal material placed in the cell to a certain definite value, e.g. .DELTA.n.d=0.5 or .DELTA.n.d=1.0, in order to prevent an occurrence of an interference fringe on the cell surface. In this situation, it may be possible to reduce the value of d when a liquid crystal material having a large .DELTA.n value is used. When the d value becomes small, an electro-optical response time (hereinafter often abbreviated to .tau.) is extremely reduced because .tau. is proportional to viscosity (hereinafter often abbreviated to .eta.) of the liquid crystal material and also to a square of the cell gap d. Thus, a liquid crystal mixture having both a suitably large .DELTA.n value and a low viscosity is very useful as a liquid crystal material for such a display device as the aforementioned AM-LCD.
Further, in order to expand a nematic range of a LC mixture, it is necessary to introduce to the LC mixture a liquid crystal compound having a high NI point and a good compatibility with other liquid crystal materials. It is familiar that a compound having a high clearing point and a large number of six-membered rings in its chemical structure is generally mixed with other LC materials for obtaining high thermal stability of the resulting mixture. But, in this case viscosity of the resulting LC mixture increases simultaneously: hence there is a limit for the introduction of the thermally stable liquid crystal compound for the purpose of increasing the thermal stability of the resulting mixture.
Still further, in order to obtain a high contrast display particularly in an AM-LCD there is a demand for a so-called highly reliable liquid crystal composition which has a high specific resistance or a high voltage holding ratio (hereinafter often abbreviated to V.H.R.).
Unexamined Japanese patent publication No. 289682/1990 discloses that a liquid crystal composition containing a benzonitrile compound having a terminal CN group is inadequate to be used as an AM-LCD material because of the low specific registance, and that a LC composition containing a difluorophenyl compound having a terminal F atom is suitable for an AM-LCD material with its large specific registance. However, a LC composition disclosed in the same publication has a defect that it cannot satisfy a quick electro-optical response of the aforementioned problem because of its small .DELTA.n value.
Although various attempts have been made to liquid crystal materials in different ways of their applications, it is the present status that a novel improvement should continually be sought to the LC materials.